Field of Disclosure
The invention relates to a voltage mode transmitter, and more particularly, to a voltage mode transmitter including a resistor ladder circuit.
Description of Related Art
Nowadays, a current mode transmitter or a voltage mode transmitter is generally used as the signal transmitter of a high-speed transmission interface. Referring to FIG. 1A, FIG. 1A is a schematic diagram showing a current mode transmitter 110 and a current mode signal receiver 120. The current mode transmitter 110 includes a current source 111 having a current I1, transistors T1 and T2 forming a differential pair, a resistor R1 and a resistor R2. The current mode signal receiver 120 includes a resistor R3 and a resistor R4, in which resistance values of the resistor R1-R4 are all equal to R. The transistors T1 and T2 are respectively controlled by a differential signal D1 and a differential signal D2. When the transistor T1 is turned on by the differential signal D1 and the transistor T2 is turned off by the differential signal D2, the current of I1 passes through the transistor T1, in which a current of I1/2 flows to the transistor T1 through the resistor R3 and a channel CH1 of the current mode signal receiver 120, so as to enable the resistor R3 in the current mode signal receiver 120 to generate a voltage drop of (R×I1)/2.
In another aspect, referring to FIG. 1B, FIG. 1B is a schematic diagram showing a voltage mode transmitter 130 and a voltage mode signal receiver 140. The voltage mode transmitter 130 includes a transistor T3, a transistor T4, a transistor T5 and a transistor T6. The voltage mode signal receiver 140 includes a resistor R5. When the transistors T3 and T6 are turned on by the differential signals D3 and D4, a current of I2 flows to the resistor R5 (a resistance value of which is equal to 2R) in the voltage mode signal receiver 140 through a channel CH1 and then flows to the transistor T6 of the voltage mode transmitter 130 through a channel CH2 to enable the resistor R5 in the voltage mode signal receiver 140 to generate a voltage drop of 2R×I2.
When the resistors of the current mode signal receiver 120 and the voltage mode signal receiver 140 have the same voltage drops, i.e., (R×I1)/2=2R×I2; I1=4×I2, to enable the voltage mode transmitter 130 and the voltage mode signal receiver 14 to have the same signal strength, the voltage mode transmitter 130 consumes a smaller current. Therefore, the voltage mode transmitter 130 has an advantage of low power consumption in comparison with the current mode transmitter 110.
In another aspect, channels between a signal transmitter and a signal receiver cause frequency-dependent attenuation, and the frequency-dependent attenuation causes Inter-symbol-interference (ISI). Referring to FIG. 2, FIG. 2 is a schematic diagram showing output signals Vo1, Vo2 passing through channels CH1 and CH2 and input signals Vi1, Vi2. Taking the voltage mode transmitter and the voltage mode signal receiver as an example, the channels CH1 and CH2 are used to connect the voltage mode transmitter with the voltage mode signal receiver, and each of the channels CH1 and CH2 is actually similar to a low-pass filter. In other words, high frequency portions of the output signal Vo1 and the output signal Vo2 of the voltage mode transmitter 130 are greatly attenuated after passing through the channels CH1 and CH2, and low frequency portions of the output signals Vo1 and Vo2 are slightly attenuated after passing through the channels CH1 and CH2. As shown in FIG. 2, when the output signals Vo1 and Vo2 of the voltage mode transmitter are square waves and pass through the channels CH1 and CH2, they are changed to the input signals Vi1 and Vi2 of the voltage mode signal receiver, in which high frequency portions of the input signals Vi1 and Vi2 has been attenuated, and a serious signal distortion is raised accordingly. Therefore, how to avoid the signal distortion of a high frequency portion of a signal is one of the current research and development topics.